Hi Dylan, I will expose what I modestly think about.
1. does (1-coefbh)=coefdh? No. I think you are confusing the clear-sky indices for the direct and diffuse components with the direct and diffuse fractions. These latter are both referred to the total global radiation coming into the ground so that, considering the reflected component negligible, the addition of the direct and diffuse fractions are always the unity. In turn, the clear-sky index for the direct horizontal component (coefbh) is the fraction of direct component attenuated by the clouds, that is, the ratio of direct beam horizontal radiation under overcast-skies to the direct beam horizontal radiation under clear-skies. The same is applied to the diffuse component. 2. does coefdh correspond to the notion of the 'diffuse fraction' which is commonly calculated via something called the 'clearness index' ? I think I have answered above. My personal opinion is that the best approach consists on using a more or less approximated turbidity coefficient of Linke (a climatological value could be the easiest choice) to get the clear-sky estimation (r.sun) and then use the ground global radiation measurement (supposing you have it, obviously) to correct the clear-sky estimation provided by r.sun. This approach can be also applied to both, the diffuse and direct components of the radiation. The problem with the components is the seldom availability of measurements. I hope to have cleared I little the question José A. > -----Mensaje original----- > De: [EMAIL PROTECTED] [mailto:grassuser- > [EMAIL PROTECTED] En nombre de Dylan Beaudette > Enviado el: lunes, 25 de junio de 2007 19:25 > Para: [email protected] > CC: [EMAIL PROTECTED] > Asunto: [GRASS-user] Use of coefbh and coefdh in r.sun > > Hi, > > After consulting with an atmospheric scientist, I have decided that it > might > be simpler to estimate 'real-sky' radiance as opposed to trying to > estimate > the Linke turbidity for the estimation of 'clear-sky' conditions. > > It looks like r.sun can use input maps: coefbh and coefdh to compute > real-sky > radiance values. I would like to be sure that I am interpreting these > inputs > correctly: > > 1. does (1- coefbh) = coefdh ? > 2. does coefdh correspond to the notion of the 'diffuse fraction' which > is > commonly calculated via something called the 'clearness index' ? > > > If this is the case, then it is not all that difficult to compute > coefdh from > pyranometer data and the output from r.sun (mode 1) > > I will update the man page with my approach, if these assumptions are > correct. > > cheers, > > PS: > a relevent references is: > > Jacovides, C.; Tymvios, F.; Assimakopoulos, V. & Kaltsounides, N. > Comparative > study of various correlations in estimating hourly diffuse fraction of > global > solar radiation. Renewable Energy, 2006, 31, 2492 - 2504 > > > -- > Dylan Beaudette > Soils and Biogeochemistry Graduate Group > University of California at Davis > 530.754.7341 > > _______________________________________________ > grassuser mailing list > [email protected] > http://grass.itc.it/mailman/listinfo/grassuser > > __________ Informacisn de NOD32 2352 (20070625) __________ > > Este mensaje ha sido analizado con NOD32 antivirus system > http://www.nod32.com _______________________________________________ grassuser mailing list [email protected] http://grass.itc.it/mailman/listinfo/grassuser
